The present invention relates generally as indicated to structure-to-soil potential difference surveys such as disclosed in prior U.S. Pat. No. 4,151,458 to William H. Seager.
Closely spaced pipe-to-soil potential surveys are conducted periodically of buried structures such as pipeline to monitor cathodic protection and to locate areas of potential problems.
Such electrical surveys require a contact to the pipe, a suitable voltmeter or potentiometer, and a means of contacting the ground. A copper-copper sulfate (Cu-CuSO.sub.4) cell is an industry standard for providing the necessary contact with the ground. Such cells or electrodes are illustrated in the above-noted Seager U.S. patent. Reference may also be had to the early Schlumberger U.S. Pat. No. 1,163,469 for a further illustration of such type of electrode. Such electrodes are usually mounted on the lower end of a pole or cane which is hand held by the surveyor.
Such electrodes have a relatively small area of contact resulting in rather high contact resistance between the electrode or reference cell and the earth. If the resistance between the reference electrode and the earth is relatively high and, therefore, represents a significant portion of the total circuit resistance, then the voltmeter in the circuit will read a significantly lower voltage value than actually exists between the reference cell and the structure which is being monitored. Generally the resistance of the metal paths including the lead wire and the metal structure are relatively small and the only two significant resistances exist at the voltmeter and at the contact between the cell or electrode and the earth. As an example, if the measuring voltmeter has an effective resistance of one million ohms and the reference cell resistance is 500,000 ohms, the voltmeter then only registers two-thirds of the actual voltage difference that exists between the reference cell and the structure in the earth. This is a particular problem in dry soils, sandy soils or rock laden areas. Readings become a small fraction of what they should be.
Moreover, when the reference cell is hand held as indicated in the noted prior art, the surveyor generally tries to force the electrode against the earth to obtain good contact. After a few kilometers of surveying in dry, sandy or rocky areas, the surveyor's arms begin to feel like a boxer's in the fifteenth round.
The problems encountered when surveying under dry conditions have long been recognized as seen, for example, in Jakosky U.S. Pat. No. 2,105,247 wherein a spray is employed to dampen the surface of the earth next to the contacts. The same inventor in U.S. Pat. No. 2,256,742 has also suggested that, in short surveys where the soil affords good surface contact, a man may be provided with contact plates attached to his feet. While the particular construction of the plates is not described, it is apparent that the contacts are not intended for areas where the soil affords poor surface contact nor can such plates provide the reduced reference electrode contact resistance which is afforded by the industry standard copper-copper sulfate (Cu-CuSO.sub.4) cell.